The present invention relates to a sputtering target comprising a refractory metal material, such as Ta and Ru, particularly for use in the manufacture of semiconductor LSIs and a method of making the sputtering target.
Conventionally, Al and Al alloys have been used as wiring materials for semiconductor LSIs. However, with the recent high integration design, minute design and high-speed design of operation of LSIs, it has been examined to use Cu which has higher electromigration (EM) resistance and higher stress migration (SM) resistance and provides low electric resistance. However, Cu readily diffuses into the SiO2 of interlayer dielectric film and also into an Si substrate and, therefore, it is necessary to enclose Cu wiring with a diffusion barrier layer. As the barrier materials for Cu, a TaN film formed by performing reactive sputtering in an atmosphere of argon and nitrogen through the use of a Ta sputtering target and a Taxe2x80x94Xxe2x80x94N film formed by performing reactive sputtering through the use of a Taxe2x80x94X alloy target are considered good. For this reason, Ta and Taxe2x80x94X alloy sputtering targets for barrier metal application for semiconductor LSIs have been developed.
On the other hand, in a DRAM and FeRAM of semiconductor memory, a Pt film formed by the sputtering of a Pt target has so far been used as a capacitor electrode. However, with further large capacity design, it is being examined to use an Ru film formed by performing the sputtering of an Ru target as a capacitor electrode.
In the manufacture of targets of the above high-melting metals or their alloys (Ta, Ta alloys, Ru, etc.), any one of a melting-plastic working process and a powder sintering process can be selected, while the powder sintering process is most suited. Reasons for this are described below.
First, although the hot plastic working of Ta is possible, it is very difficult to make crystal grains uniform and fine. According to results of an investigation performed by the present inventors, coarsened crystal grains of a target are one of the major causes of generation of particles during sputtering. Recently, addition of a third alloying element to Taxe2x80x94N has been proposed in order to improve the barrier property of a Taxe2x80x94N film. Si and B are mentioned as such alloying elements and it is said that a Taxe2x80x94Xxe2x80x94N film formed by the reactive sputtering of a Taxe2x80x94X target (X: alloying element such as Si and B) becomes amorphous, thereby improving the barrier property. However, a Taxe2x80x94X alloy target raises a problem that plastic working is impossible due to the segregation by solidification and the formation of intermetallic compounds.
On the other hand, in the case of Ru, manufacture by plastic working is impossible because this metal does not have plastic workability. Therefore, it can be said that the superiority of the powder sintering process, including an advantage of yield improvement in the near-net-shape manufacture of targets, is clear as a method of making high-melting metal targets of Ta, Taxe2x80x94X alloys and Ru.
Incidentally, with the recent high integration design of LSIs and minute design of devices, requirements for a reduction of impurities in materials for thin films have become very severe. In particular, for transition metals (Fe, Ni, Cr, etc.) and alkali metals (Na, K, etc.) which are considered to have a great adverse effect on the performance of devices, it is required to reduce such impurities to the order of ppb, and for radioactive elements (Th, U, etc.) to the order of ppt. Furthermore, for other low-melting metal impurities also, it is required to lower their concentrations and, as a result, it is necessary to increase purity to not less than 99.999%. In addition, in order to improve the thermal stability of barrier films, the interface electric characteristic of DRAM capacitor electrode films, etc., it is also required to lower oxygen concentrations to not more than 100 ppm.
Ta powders that can be industrially supplied are conventionally obtained by an ingot crushing process after performing the EB melting of a low-purity Ta raw material, and their purity is only a level of 4N at the most. On the other hand, the following method, for example, is adopted as a process for industrially making Ru. Caustic potash and potassium nitrate are added to crude Ru, thereby converting Ru into soluble potassium ruthenate. This salt is extracted in water and is heated during chlorine gas injection under the formation of RuO4, which is then collected in dilute hydrochloric acid containing methyl alcohol. This liquid is evaporated and dried, and is then calcined in an oxygen atmosphere to form RuO2, with the result that Ru metal is finally obtained by reduction under heating in hydrogen. Commercial Ru powders made by this method contained low-melting metal impurities, alkali metals, and residues of halogen elements such as Cl and hence could not meet the purity required of capacitor electrode films. Moreover, powders made by this method were coral-like porous agglomerates and had very low packing densities in the case of sintering.
In order to increase the purity of Ta and Ru targets, there have been proposed methods of refining the above raw material by EB melting, more concretely, a method which involves plastic working of an Ta ingot obtained by the EB melting and a method of machining an Ru ingot obtained by the EB melting into a target in a casting condition thereof. For example, JP-A-3-197940 discloses a method of plastically working an Ta ingot obtained by EB-melting. Also, JP-A-6-264232 discloses a method of performing plastic working and heat treatment of Ta after the EB melting thereof. Further, JP-A-11-61392 discloses a method of machining an ingot obtained by the EB melting of an Ru raw material and using it in a casting condition thereof.
High purity may be realized by using the methods disclosed in the above literature. In those cases, however, as mentioned above, there is a fear of causing a coarse or nonuniform of the microstructure at the stage of plastic working of an ingot. Further, with a material in a casting condition, the presence of a large number of pores and casting defects cannot be neglected. In addition, in the melting methods, it is impossible to perform near-net-shape forming and the yield of noble metals is low. In other words, it can be said that the melting methods proposed in the above literature are an unavoidable choice because high purity and low oxygen concentrations could not be realized in the powder sintering method.
In general, it is difficult to sinter the refractory metals (more concretely, metals each having a melting point higher than that of iron) to high density in order to increase the density of a sintered compact, pressure sintering is one of effective methods. Because metal powders are filled into a capsule and then the capsule with packing powders is sintered, the packing condition of a raw material powder is an important factor. In hot isostatic press (HIP), increasing the packing density accelerates an increase in the density of a sintered compact and reduces abnormal shrinkage during sintering and sinter cracks, resulting in an increase in yield. In other words, in performing sintering under pressure, packing a raw material powder at a high density and uniform packing bear an important meaning. It is well known that the spheroidizing treatment of a raw material powder is effective in realizing such high packing density and uniform packing. However, in case of using a crushed Ta powder and a coral-like Ta powder, the packing density is low and, therefore, the optimization (spheroidizing) of these powder shapes is also an important problem in the sintering technology of targets.
As a method for realizing the spheroidizing of a high-melting metal powder, JP-A-3-173704 discloses a method of producing a spherical Ta powder by Plasma Rotating Electrode Process (PREP) treatment, i.e., by bringing a thermal plasma into contact with a rotating electrode and thereby causing an electrode material to melt and splash. Under this method, however, the thermal plasma is given with the purpose of only spheroidizing performed by heating and melting, and the effect of purification of powder cannot be expected.
Therefore, an object of the invention is to provide a method of making a high-melting metal powder which has high purity and which is excellent in compacting and sintering, and particularly a method of making a spherical powder made of Ta, Ru, etc. having a higher melting point than that of iron. Another object of the invention is to provide a target made of high-melting metals or its alloy, which is producted by sintering these powders under pressure and which has high purity and low oxygen concentrations and besides shows high density and a fine and uniform micro-structure.
In order to attain the above objects, the present inventors have conducted research energetically and found out that by applying thermal plasma treatment to a raw material powder, it is possible to spheroidize a high-melting metal powder and, at the same time, to obtain high purity and low oxygen concentrations. Further, the inventors have found out that by performing sintering under pressure through the use of this powder which is spherical and has high purity and a low oxygen concentration, it is possible to increase packing density, with the result that it is possible to obtain a sintered powder compact suitable for sputtering targets, which has high purity and a low oxygen concentration and besides shows a high density and a uniform and fine micro-structure.
In the method of making a target according to the invention, by introducing a powder mainly composed of a high-melting metal into a thermal plasma flame, refining and spheroidizing are performed and an obtained powder is sintered under pressure. As a result of this process, it is possible to obtain a sputtering target which has high purity and a low oxygen concentration and besides shows high density and a uniform and fine micro-structure.
Further, in the method of making a target according to the invention, a powder is introduced into a thermal plasma into which hydrogen gas has been introduced. As a result of this process, it is possible to obtain a sputtering target having chemical composition which has high purity and a low oxygen concentration and besides shows high density and a uniform and fine micro-structure.
Also, in the method of making a target according to the invention, the sintering under pressure is hot isostatic pressing. As a result of this process, it is possible to obtain a sputtering target which shows high density and a uniform and fine micro-structure.
The sputtering target according to the invention comprises a sintered powder compact with a relative density of not less than 99%, a purity of not less than 99.999% and an oxygen concentration of not more than 100 ppm. As a result of this matter, a thin film obtained by sputtering through the use of this target has high purity and is uniform so as to improve the reliability of products.
Also, the sputtering target according to the invention is obtained by performing the sintering under pressure of a powder obtained by introducing a powder material mainly composed of a high-melting metal into a thermal plasma. As a result of this process, a thin film obtained by performing sputtering through the use of this target has high purity and is uniform, improving the reliability of products.
Also, the sputtering target according to the invention is obtained by introducing a powder into a thermal plasma into which hydrogen gas has been introduced. As a result of this, a thin film obtained by performing sputtering through the use of this target has high purity and is uniform so as to improve the reliability of products.
Also, in the sputtering target according to the invention, the shape of particles of the powder introduced for sintering under pressure is spherical or analogous to a sphere. As a result of this, the target shows high density and a uniform and fine micro-structure, and the uniformity of a thin film obtained by performing sputtering through the use of this target increases.
Further, in the sputtering target according to the invention, the above high-melting metal material is Ta. As a result of this matter, it is possible to obtain a Ta target which has high purity and a low oxygen concentration and shows high density and a uniform and fine micro-structure, and by using this Ta target, it is possible to obtain a Ta thin film which has high purity and is uniform.
Further, in the sputtering target according to the invention, the above high-melting metal is Ru. As a result of this matter, it is possible to obtain an Ru target which has high purity and a low oxygen concentration and shows high density and a uniform and fine micro-structure, and by using this Ru target, it is possible to obtain an Ru thin film which has high purity and is uniform.
Further, the high-melting metal powder according to the invention has a purity of not less than 99.999% and an oxygen concentration of not more than 100 ppm and the shape of particles of the high-melting metal powder is spherical or analogous to a sphere. By performing pressure sintering with the use of this powder, the packing density of the powder increases and it is possible to obtain a formed powder compact which shows high density and a uniform and fine micro-structure.
Further, the high-melting metal powder material according to the invention is obtained by introducing a powder mainly composed of a high-melting metal into a thermal plasma. As a result of this process, the obtained high-melting metal powder becomes a powder of spherical particles which has high purity and a low oxygen concentration as a chemical composition, and by performing pressure sintering through the use of this powder, it is possible to obtain a formed powder compact which has high purity and a low oxygen concentration and besides shows high density and a uniform and fine micro-structure.
Further, the high-melting metal powder according to the invention is obtained by introducing a powder into a thermal plasma into which hydrogen gas has been introduced. As a result of this process, the obtained high-melting metal powder material becomes a powder of spherical particles which has high purity and a low oxygen concentration, and by performing pressure forming through the use of this powder, it is possible to obtain a formed powder compact which has high purity and a low oxygen concentration and besides shows high density and a uniform and fine micro-structure.
As mentioned above, the greatest feature of the invention resides in the fact that a powder mainly composed of a metallic material with a higher melting point than iron, particularly Ta or Ru, is introduced into a thermal plasma, thereby to obtain a high-melting metal powder material which has high purity and a low oxygen concentration and the shape of whose particles is spherical or analogous to a sphere. When a radio-frequency (RF) thermal plasma is specifically selected from among thermal plasmas and used as a heat source, the range of the thermal plasma widens and it is possible to suppress the contact of the powder with other substances during treatment. This is most favorable for obtaining high purity.
Further, by introducing hydrogen into a thermal plasma gas, it is possible to remarkably improve the evaporation of impurities and the effect of the reduction of oxygen owing to the generation of ions and excited atoms of hydrogen.